Controlling the selectivity of the hydrogenolysis of polyamides catalysed by ceria-supported metal nanoparticles

Catalytic hydrogenolysis is a promising approach to transform waste plastic into valuable chemicals. However, the transformation of N-containing polymers, such as polyamides (i.e. nylon), remains under-investigated, particularly by heterogeneous catalysis. Here, we demonstrate the hydrogenolysis of various polyamides catalysed by platinum-group metal nanoparticles supported on CeO2. Ru/CeO2 and Pt/CeO2 are both highly active but display different selectivity; Ru/CeO2 is selective for the conversion of all polyamides into water, ammonia, and methane, whereas Pt/CeO2 yields hydrocarbons retaining the carbon backbone of the parent polyamide. Density functional theory computations illustrate that Pt nanoparticles require higher activation energy for carbon−carbon bond cleavage than Ru nanoparticles, rationalising the observed selectivity. The high activity and product selectivity of both catalysts was maintained when converting real-world polyamide products, such as fishing net. This study provides a mechanistic basis for heterogeneously catalysed polyamide hydrogenolysis, and a new approach to the valorisation of polyamide containing waste.

Introduction lines 57-60: Some work is missing on polyamide recycling via hydrogenative depolymerization in presence of NH3 which should be cited and taken into account: GB1179706 and DE1695282A1 (hydrogenation of polyamides in the presence of NH3 using Ru-and Ni catalysts) ACS Sustainable Chem.Eng.2022, 10, 3048-3056 (hydrogenation of polyamides in the presence of NH3 using Nb2O3 and a Ru catalyst).
Line 60-63: In the before mentioned paper, also polyamides containing up to 30% carbon black was depolymerized.Please take into account.
Line 85-87 and lines 191-195.See the supporting information page tables S3 and scheme S3 of ref 20.There hydrogenative C-heteroatom cleavage in in polyamide was observed with a heterogeneous Ru catalyst and hexane formed.Generally, and considering the other work on polyamide depolymerization: Here the authors are making methane, hexane or dodecane.In other work, one gets at least functional molecules back as amines or alcohols.With methane or hexane, on cannot do that many thing besides just burning it or making syngas out of it (and then go into the syngas value chain).But why then not just burn the polyamide to get the heating value or making syngas rather then adding a high-pressure hydrogenation step before with precious metal catalysts and losing all functionalities?The authors must point more out, where is the significant advantage of this approach compared to others, where one would get more functional small molecules back.

Supporting information, Table 1:
The old work on the heterogeneous hydrogenation of polyamides with Ru-and Ni catalysts in the presence of ammonia (GB1179706 and DE1695282A1; both patents accessible via espacenet from the EPA) should also be taken into account here.
The recent work in ACS Sustainable Chem.Eng.2022, 10, 3048-3056 should also be taken into account here.
The entry with the hydrogenation (homogeneoulys)is not correct.Ref 6 is cited, but in ref 6, polyamides where hydrogenated in DMSO, not THF.And in ref 6 also PA66 is hydrogenated to the diol and diamine, see entry 4 in table 2 in this ref.Not only 6-amnio-1-hexanol.THF was used in the system presented in ref 20 of the manuscript.By the way, in ref 20, the products obtained are 1,6-hexamehtylenmediamien and 1,6-hexandiol in yields up to 78% for a model PA (see table 2 in ref 20).This should also be reflected here.
Reviewer #2 (Remarks to the Author): The manuscript titled "Controlling the selectivity of the hydrogenolysis of polyamides catalysed by ceria supported metal nanoparticles" reports the selective hydrogenolysis of polyamide models and difficultto-recycle 'real-world' polyamide waste using M/CeO2 catalysts with high activities.Density functional theory has been used to rationalize selectively, and the effect of hydrogenolysis conditions on catalyst activity and selectively have been investigated.Overall, this is a very thorough, logical, and wellconducted study in an important field of research.Therefore, I recommend publication in Nature Communications after some minor corrections and suggestions detailed below: 1.The resolution of the TOC and images throughout the manuscript and supplementary information could be improved.
3. The catalyst characterization and theoretical studies relate very nicely to the experimental data, with detailed explanations provided.Have any attempts been made to look at the acidity of the catalysts?Differences in acidity could potentially explain the differences in activity observed for the Ru/MxOy catalysts.Additionally, H2 chemisorption could also be used to quantify the surface area and dispersion of the metal NPs on the surface.Note: these are suggestions, not requirements for publication.
4. The catalyst screening section states: "A control reaction was performed to determine the thermal stability of N-hexylhexanamide under the reaction conditions.The control reaction indicates that the substrate undergoes low levels (6%) of thermal decomposition, confirming that the observed hydrogenolysis activity was due to the activity of the metal catalysts."The authors should consider mentioning here that 24.4% conversion is observed with just the CeO2 support, with the conversion increasing when metal NPs are on the surface.The current sentence implies that all the observed activity is from the metal NPs, whereas some activity may also come from the support.8. It's great that these catalysts are able to break down real-world plastic waste, PA blends, and PA composites.It would be beneficial to mention some of the uses and value of the products (methane, nhexane, n-dodecane etc.) in the conclusions.
Reviewer #3 (Remarks to the Author): Revisions: The manuscript demonstrates the hydrogenolysis of various PAs catalysed by platinum-group metal nanoparticles (NPs) supported on ceria (CeO2).Ru/CeO2 was found to be the most active in catalysing hydrogenolysis of PA materials rapidly into water, methane and ammonia, whereas Pt/CeO2 yields hydrocarbons retaining the carbon backbone of the parent PA.The DFT computations was also performed to rationalize the observed selectivity.The work is meaningful, and can be accepted after a major revision.My detailed comments are as follows: In page 4, the authors mentioned that "there have been limited studies exploring the cleavage of carbon−heteroatom bonds in PAs".However, there are numerous experimental and theoretical studies have been reported about the homogeneous hydrogenolysis of PA and polyamides.Some literatures (e.g.Chem.Sci.,2021, 12,10590; JACS Au 2021, 1, 517−524; Inorg.Chem.2022, 61, 14662−14672)  should be introduced.What is more, the advantages of Ru/CeO2 and Pt/CeO2 over the homogeneous catalysts must be described in detail.
In Fig. 2, the N-hexylhexanamide undergo the C-O hydrogenolysis to form a secondary amine intermediate, and the C-N hydrogenolysis to form a N-terminal amine.However, the amides are widely reported to undergo the hydrogenolysis to provide N-terminal amines and alcohols (ACS Catal.2020, 10,  5511−5515; J. Am.Chem.Soc.2019, 141, 12962−12966; Nat.Commun.2015, 6, 6859), and the secondary amine intermediates are not involved.The mechanism in Fig. 2 could not be right.
The discussion of chemoselectivity is based on the reactions of CH3CCH in scenario II of Fig. 4, but the theoretical study does not provide the pathway for the formation of CH3CCH intermediate.Otherwise, the experiment should prove the formation of CH3CCH.
In this manuscript, the hydrogenation of amides and polyamides to hydrocarbons as methane, hexane or dodecane using different precious metal based heterogeneous catalysts is presented.The recycling of polymer waste is a topic of high current interest to address a global challenge.Therefore, ne approaches to utilize plastic waste are highly welcome.From a catalysis point of view (synthesis, characterization, reactivity, recyclability), the work seems to be solid.Nevertheless, there are certain concerns about the usability of this approach and how the state of the art is taken into account.Therefore acceptance for nature communications is not recommended.
We thank the reviewer for their positive appraisal of the work conducted and also their concerns which are commented on further below.

Introduction lines 57-60: Some work is missing on polyamide recycling via hydrogenative depolymerization in presence of NH3 which should be cited and taken into account: GB1179706 and DE1695282A1 (hydrogenation of polyamides in the presence of NH3 using Ru-and Ni catalysts). ACS Sustainable Chem. Eng. 2022, 10, 3048-3056 (hydrogenation of polyamides in the presence of NH3 using Nb2O3 and a Ru catalyst).
We thank the reviewer for drawing our attention to these reports of hydrogenative ammonolysis, and we have added these references to the introduction (References 21-23).
However, hydrogenative ammonolysis is fundamentally different to our approach, the former uses an excess of ammonia, whereas the approach we report generates ammonia, which can be easily separated from the hydrocarbon products.Moreover, in the present work, mixed plastic waste (containing polyamides, polyolefins, and structural additives) may be transformed, which excludes the need for the challenging task of plastic waste separation.Overall, we believe that all these methods are important and are potentially viable in different situations.

Line 60-63:
In the before mentioned paper, also polyamides containing up to 30% carbon black was depolymerized.Please take into account.
As requested, we have accounted for this in both the introduction and Supplementary Table 1: Two of these studies investigated the conversion of PA-blends (containing a mixture of different PAs) or PA composites 13,21 , which contain PA blended with other materials, such as carbon fibre or glass.The heterogenous catalysts used in ref 20 (ChemSusChem 2021, 14 (19), 4176-4180) were employed as controls, used to confirm that their reaction system is homogenous.We have now commented on this work in the introduction:

Supplementary
Despite the various studies which focus on the design of heterogeneous catalysts for selective C−O bond hydrogenolysis, [39][40][41] there have been limited studies exploring the cleavage of carbon−heteroatom bonds in PAs.However, a supported-Ru catalyst was used to confirm the homogeneity of a Ru-complex used to catalyse PA hydrogenation 20 .We have changed the colour scheme of all the bar columns following the reviewer's suggestion.

Generally, and considering the other work on polyamide depolymerization:
Here the authors are making methane, hexane or dodecane.In other work, one gets at least functional molecules back as amines or alcohols.With methane or hexane, on cannot do that many thing besides just burning it or making syngas out of it (and then go into the syngas value chain).But why then not just burn the polyamide to get the heating value or making syngas rather then adding a highpressure hydrogenation step before with precious metal catalysts and losing all functionalities?
The authors must point more out, where is the significant advantage of this approach compared to others, where one would get more functional small molecules back.
We have added further discussion to better emphasize the advantages of our approach, emphasised the formation of ammonia and the uses of the hydrocarbon products.The difference in C-C hydrogenolysis ability between Pt and Ru, which controls selectivity, is fundamentally interesting.Furthermore, the n-hexane and n-dodecane produced can be used as solvents.We have added a sentence to highlight the uses of the alkane products: The Ru/CeO2 catalyst leads to near quantitative conversion for all PA resins, with methane being the primary hydrocarbon product in all cases, obtained in up to 99% yield for PA-12 and PA-612.This provides a power-to-methane process which can potentially be directly utilized by existing natural gas networks 31 .In contrast, the Pt/CeO2 catalyst results in slightly lower conversion rates, with 88% conversion for PA-6, 78% for PA-66, 98% for PA-12 and 99% for PA-612.Similar to the previous studies on model amides, Pt/CeO2 also showed a preference for the formation of higher-order alkanes, such as n-hexane or n-dodecane, depending on the length of the carbon backbone in the polymer.These longer chain hydrocarbons have uses as non-polar solvents with multiple applications including in printing, adhesives, textile manufacture etc., and in the case of n-dodecane, as a jet fuel 65 .
We have emphasized the formation of ammonia in the discussion: The Ru/CeO2 and Pt/CeO2 catalysts are able to transform PA composites or blends and a PA-66 fishing net into high yields of methane or higher-order hydrocarbons, together with ammonia.
We have emphasised in the discussion the significant advantage of the M/CeO2 catalysts which is its ability to valorise mixed plastic waste: In addition, the ability to selectively convert mixtures of PA and polyolefin waste into two easily separated products, i.e., methane and ammonia, is of immense potential value, as the separation of waste plastics is challenging.
Also see response to reviewer 3, comment 2 where we specifically highlight the advantages of our heterogeneous catalysts.

Supporting information, Table 1:
The old work on the heterogeneous hydrogenation of polyamides with Ru-and Ni catalysts in the presence of ammonia (GB1179706 and DE1695282A1; both patents accessible via espacenet from the EPA) should also be taken into account here.
The recent work in ACS Sustainable Chem.Eng.2022, 10, 3048-3056 should also be taken into account here.
As described above, we have taken these studies into account in both the main text (References 21-23) and the SI (Reference 7).).This should also be reflected here.

Figure 2 ,
Figure 2, figure 3, figure 5 and figure 6: The authors should use other colors for the columns.It is very difficult to get, what is which column.
5. Fig 2b.The scheme is missing H2O as a product on the first arrow.6.Catalyst screening: Reference Supplementary Figure2cwhen discussing PdCl2.7.FiguresS11 and S12: add the overall conversion to the plots (similar to Figures2 and 3in the manuscript).

Figure 2 ,
figure 3, figure 5 and figure 6: The authors should use other colors for the columns.It is very difficult to get, what is which column.